you dont have piercing fireballs^^ you have exploding errors which can be empowered by enchant and fire mastery. but since shooting arrows has nothing to do with fireballs, the damage of the arrows does not get increased by synergies to fireball.
also consider that your attack speed will probably be lower than your cast speed by quite a margin, so this will have to be matched by the piercing effect (have to do a comparison here, go check the numbers)